The present invention relates generally to electromagnetic devices such as solenoids, relays, clutches and the like and more particularly to control circuits for these devices which regulate the application of electric current thereto.
Electromagnetic devices such as solenoids often require considerably greater magnetic flux during the initial actuation or pull phase when the armature is moved from a first, rest position to a second, actuated position and they require considerably less flux to hold the armature in the second, actuated position. A coil and current optimized for the pull phase is not suitable for the hold phase. It will overheat and waste power. Various inventions of the prior art are directed to the problem. Some employ two different coils, a pull coil and a hold coil. Power is first applied to the pull coil alone or with the hold coil. When the armature has moved, the pull coil is deactivated and power is applied, or continued to be applied, to the hold coil. Switching may be accomplished through mechanical action of the armature on a switch, or a timed initial power pulse may be applied to the pull coil while continuous power is applied to the hold coil. When applied to a single coil electromagnet an initial timed pull pulse may be superimposed on a continuous hold voltage.
If the pull coil is optimized for a particular size and duration of voltage pulse, it will have just the amount of metal in the wire coil to carry the necessary current to provide the necessary flux with just enough insulation to permit correct winding and heat dissipation.
If the device is energized again before the coil has cooled down from the prior operation, it may burn out. To overcome this problem, the coil is made with a safety factor, but is now no longer optimized for its intended function. It may be larger, heavier and more expensive. Furthermore, small line voltage fluctuations may actuate the circuit and heat up the coil. The designer must make a compromise between reliability and optimum operation. Since replacement of a burned out coil may be very expensive in time, money and reputation, most instruments are manufactured with solenoids that are not optimized for their intended function.